In conventional heat-treatment plants, especially in plants used for pasteurizing products, control is effected according to the number of pasteurization units transferred (PU control). The pasteurization units, which are abbreviated hereinbelow with PU, are calculated according to the following formula:
            ⅆ      PU              ⅆ      t        =      10    ⁡          [                                    t            p                    -                      T            ref                          z            ]      In this formula,    Tp stands for the product temperature, the pasteurization parameters being normally    Tref=60° C. and    z=6.949468.
Depending on the product in question, a certain pasteurization degree is determined, which is necessary for achieving a sufficient preservation of the product, without impairing it. According to this pasteurization degree, the pasteurization units to be absorbed by the respective product are determined; the transfer of these pasteurization units must be guaranteed by a suitable temperature control of the plant in question.
An apparatus for pasteurizing products, which is manufactured by the applicant, is known from European patent 0 437 499 B1. By means of this known apparatus, excessive or insufficient pasteurization of the products is avoided, when the plant is stopped during the heat treatment, and the power consumed is reduced simultaneously. In the known apparatus, this is achieved by crosswise rerouting spray liquids, which flow off from zones with different temperatures, in such a way that cooler products in a pasteurization area are used for cooling warmer products at a different location in said apparatus by means of heat compensation of the spray liquid in this area. An additional supply of cold spray liquid or a discharge of warm spray liquid and the resultant power losses are avoided in this way.
This known apparatus is controlled exclusively according to the number of pasteurization units transferred, which must reach a specific reference value, i.e. a specific pasteurization degree within close tolerances.
DE 36 37 661 C2 discloses a method for pasteurizing products, with the aid of which a power-saving and reliable pasteurization is to be made possible even if malfunctions occur when the receptacles to be pasteurized pass through the plant. In order to achieve this, a prepasteurization zone and a pasteurization zone are subdivided into controllable elementary zones; in each elementary zone, at least one reference receptacle is observed and the number of pasteurization units absorbed by said receptacle is calculated. When a specific reference value is exceeded or not reached, cooling or heating of the respective zone is initiated, whereby an optimum, variable adaptation of the spray temperature can be effected, when the speed of passage is known.
Also in the case of this known method, process control is effected exclusively in accordance with the number of pasteurization units transferred.
An apparatus of the type mentioned at the beginning is produced and marketed by the applicant.
Just as the above-described plant and the above-described method, the control unit of this known apparatus supports process control according to a reference value of a controlled variable, in particular according to a reference value of the pasteurization units.
In addition, the known apparatus also permits one or a plurality of other criteria to be supervised, e.g. the lethal temperature, i.e. the temperature that has to be reached for killing germs. On the basis of the result of this supervision, a heat treatment is carried out after the normal pasteurization process, if necessary, e.g. if a product does not reach a certain temperature level, i.e. the germ-killing point, prior to leaving the last heat treatment zone. A conveyor belt transporting the product in question is then stopped and the thermal after-treatment is carried out until the supervised criterion has been fulfilled; this results in considerable downtimes of the plant.
This intervention in the process sequence is a control, i.e. there is no closed signal circuit in which a controlled output magnitude acts back on a controlling input magnitude. In contrast to a closed-loop control, an open process, which does not comprise an adaptation of an actual magnitude to a reference magnitude, takes place in the known apparatus.
Known pasteurization plants have, on the whole, only been controlled according to a single reference criterion up to now; this criterion is normally the pasteurization degree. It is also known to supervise, in addition to the pasteurization unit control, other criteria, e.g. the killing-point temperature; the supervision result is, however, not incorporated in the control, but is only used for carrying out a subsequent heat treatment, if necessary.
It follows that, if a criterion other than the control criterion is not fulfilled in the sequence of normal pasteurization steps carried out in the case of the known control processes, an aftertreatment has to be carried out until the respective criterion is fulfilled. This results in undesired downtimes of the plant.